The present invention relates to a control system for four-wheel drive vehicles, and more particularly to a system for controlling the torque capacity of a transfer clutch for four-wheel drive vehicles.
A four-wheel drive vehicle of the type in which a transfer clutch is provided for transmitting the output torque of a transmission to auxiliary drive wheels is known. In such a vehicle, a power transmission system can be converted from a two-wheel drive system to a four-wheel drive system by engaging the transfer clutch. Meanwhile, when the vehicle negotites corners, braking phenomenon called "tight corner braking" will occur at a sharp corner. This is caused by greater radious of turning of the front wheels than that of rear wheels and therefore tendency to rotate faaster than the rear wheels, so that a large torsional torque occurs in the system. Japanese Patent Application Laid Open 57-84227 discloses a system which is intended to eliminate the braking phenomenon. In the disclosed system, a four-wheel drive power transmission system is converted to a two-wheel drive power transmission system when a vehicle turns a sharp corner. However, although such a system prevents the tight corner braking at sharp corners by the two-wheel drive vehicle system, characteristics of the four-wheel drive vehicle such as driveabilities on slippery roads, uphill driving and other are not effected.
In order to prevent the tight corner braking while keeping a four-wheel drive power train, it is preferable to employ a friction clutch as a transfer clutch and to control the torque capacity of the transfer clutch so as to slip the clutch in accordance with driving conditions of the vehicle. Further, in order to control the torque capacity, it is advantageous to reduce the torque capacity of the tranfer clutch in accordance with the input torque of the clutch. Considering the tight corner braking, at low vehicle speed and large steering angle tight corner brakeing force becomes large. Generally, in a very low vehicle speed range, a main clutch for transmitting the output of the engine to a transmission is kept in partial engagement. Accordingly, in partial engagement state of the main clutch, it is necessary to detect the torque transmitted by the main clutch and to calculate the torque capacity based on the transmitted torque. When the main clutch is entirely engaged, the torque capacity may be calculated based on engine torque.